How do relative and radioactive dating work

He also found that certain animals were in only certain layers and that they were in the same layers all across England.

Due to that discovery, Smith was able to recognize the order that the rocks were formed.

A similar situation with igneous rocks occurs when xenoliths are found.

These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.

however, this process is not enough to allow the layers to change their positions.

This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.

Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age (i.e. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another.

Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials.

The only disturbance that the layers experience is bioturbation, in which animals and/or plants move things in the layers.

Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault.

Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.

A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." The principle of intrusive relationships concerns crosscutting intrusions.

In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock.

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